Start-up problems are very common in amplifiers, especially where the input signal common mode is not well driven. This may cause the amplifier to stay in an incorrect operating point, causing significant or even complete performance loss. Start-up problems may result from different charging rates for various internal nodes that may result in the amplifier locking up when first turned on instead of settling into an expected quiescent state. Almost every differential amplifier includes a common mode feedback circuit. This feedback circuit includes a separate small amplifier and tries to ensure the operating voltages of different nodes in the main amplifier are at their desired levels.
Typical existing solutions to the start-up problem may use a start-up circuit, but sometimes it interferes with normal circuit operation. Some start-up circuits may need external digital control to turn them on and off. Sometimes a start-up circuit is not sufficient to ensure correct start-up across process and temperature variations. In that case, it may need to be overdesigned but then may cause performance degradation as it starts interfering with normal operation. 100% common mode feedback may ensure start-up, but achieving stability is difficult and may be nearly impossible in many situations. While 100% common mode feedback may ensure start-up, lack of stability can cause unwanted oscillations and total performance failure in amplifier operation.
Other features of the present embodiments will be apparent from the accompanying drawings and from the detailed description that follows.